Current practice in x-ray Raman, fluorescence and other spectrographic methods involves the use of sample pelletization. The advantages of sample pelletization include 1) enhanced sample-to-sample uniformity, 2) increased density of sample material and, accordingly, increased strength of emitted radiation, 3) exclusion of contaminant materials, such as air (resulting in reduced spurious emissions on account of such contaminant materials), and 4) compaction of the sample into a single undispersable tablet resulting in greatly reduced contamination of the chamber within which the sample is irradiated.
Current systems for pelletizing samples for spectrographic analysis generally use a multi-part cylindrical die into which the material to be pelletized is loaded. The loaded die is then placed in a pelletizing apparatus for compaction of the sample.
Generally, a pelletizing apparatus comprises a hydraulically-operated platen and a fixed abutment. The loaded die is placed between the platen and the abutment and the user causes the apparatus to advance the platen toward the abutment, thus applying pressure to the die and compressing the sample within into a pellet. Desired pressures, often on the order of tons, are achieved by hydraulic means, with the desired pressure being realized by the pumping of hydraulic fluid in a hydraulic circuit. In accordance with current practice, when the desired pressure is reached, the operator of the apparatus is directed to stop the operation of the pump which causes the platen to advance toward the abutment, and maintain pressure for a desired period of time. The operator then slowly manually releases the pressure in the hydraulic system, which drives the platen, through the use of a hand operated valve.
In order to protect against damaging the die, the prior art pelletizing apparatus is generally provided with a pressure relief valve which may be manually set by the operator to a pressure which, even in the event of a malfunction or operator error, will be the maximum pressure to which the die will be subjected. Such pressure is generally keyed to the physical limits at which the die can be operated and will usually be higher than the actual operating pressure during the pelletizing process
While the raising of pelletizing pressure to a desired value, holding for a desired time and release of pressure appears to be a simple and straight forward process; it actually has a number of problems which have been routinely tolerated for many years. For example, after the operation of the pump is stopped, often due to factors such as wear or the like, the pressure in the system will be slowly released. This affects the quality of the compacted sample because it has not been subjected to the desired high pressure for a long enough period of time. In addition, after the required pressing period has been undergone by the sample, it has been discovered in accordance with the invention, that the release of pressure through the actuation of the manual release valve, even in the case of highly skilled operators, often results in no change in applied pressure during an initial phase of the release and, as the valve continues to be opened wider, a relatively abrupt change in pressure. This abrupt change often results in fracturing of the pelletized sample, thus rendering it useless. One approach to this problem has been to set the pressure relief valve to the desired compacting pressure; however, this approach is clumsy and difficult to control.